1. Field of the Invention
The present invention concerns microwave tubes provided with at least one coaxial part mounted by cold fitting into a coaxial envelope.
The invention can be applied chiefly to the helical delay lines of travelling-wave tubes as well as to the guns or collectors of longitudinal-interaction microwave tubes.
2. Description of the Prior Art
A travelling-wave tube is formed by the association of a long and thin electron beam and a delay line designed to guide a microwave that has to be amplified. This delay line is often helix-shaped. It is metallic. It is kept centered by at least three dielectric spacers in the form of rods and then introduced into a sheath-shaped metal envelope. The helix may be formed by several successive sections. Attenuations may be deposited on the rods, at two ends of facing helical sections. This makes it possible to prevent the tube from oscillating, and that increases its gain.
The electron beam is produced by a gun placed at input of the delay line, and it is collected in a collector placed at output of the delay line. A focusing device surrounds the delay line so as to make the electron beam converge.
The sheath often has the shape of a cylinder generated by revolution. It is coaxial with the helical delay line. The sheath may be formed by successive metal rings. It enables a high vacuum to be maintained within the tube, and can also act as a focusing unit.
When the travelling-wave tube works in continuous mode, the helix is greatly heated. This heating is due to the power released by the electrons of the beam which strike the helix, and to the Joule's effect losses from the helix. This heating is related to the mean power level of the tube.
In order to limit this heating, it becomes necessary, firstly, to choose suitable materials for the helix, the rods and the sheath and, secondly, to provide for excellent contact between the helix and the rods, and between the rods and the sheath
The helix is often made of tungsten or molybdenum, the rods are made of boron nitride, alumina or beryllium oxide, and the rings of the sheath are made alternately of iron and stainless steel.
At present, there are two main methods used to mount the helix surrounded by the rods in the sheath.
The first method consists in force-fitting the helix/rods assembly into the sheath. This method calls for very precise control of the dimensions of the parts to obtain satisfactory clamping or gripping. Their
have to be extremely low, of the order of 5 to 7 micrometers.
The second method that is frequently used consists in heating the sheath so that it expands and then in inserting the helix/rods assembly within it. The clamping is obtained upon cooling. Under heat, the clearance needed to enable the insertion of the helix/rods assembly is in the range of 0.02 to 0.03 millimeters. However, this method has serious drawbacks.
Its implementation is lengthy and difficult. The sheath has to be heated to about 700 degrees C. The heating operation takes several hours, and so does the cooling operation. When the helix/rods assembly is inserted into the heated sheath, there is a risk that the attenuations deposited on the rods will be damaged by oxidation. As compared with the first method, this method calls for relatively strict tolerances of parts.